1. Field of the Invention
The present invention is in the field of spiral thread inserts and more particularly is directed to an improved driving tool for inserting the same.
2. The Prior Art
As is well known, helical thread inserts are becoming increasingly employed as a means for repairing stripped, worn, or damaged threads as well as presenting a hardened stainless steel thread liner in original equipment applications where frequent threading and unthreading are expected. Briefly, where the threads in a bore are stripped or worn, repair is effected by drilling out the bore to remove remnants of the damaged threads, thereafter tapping the drilled out bore and then inserting in the tapped bore a spiral insert, the outer diameter of which is intimately engaged in the threads of the retapped bore, the inner diameter of the spiral insert providing a threaded bolt receiver portion of the same size and pitch as that presented by the original threading of the bore.
In order to assure that the spiral inserts remain in position within the tapped bore, the spiral is, prior to insertion, of a somewhat larger diameter than the bore. Thus, when it is driven into position by rotation in the tapped bore, substantial torque must be asserted on the insert, resulting in its radial inward compression.
The typical installation tool employed to position spiral inserts has comprised an elongate, hardened metal cylinder having a drive end and a torque applying end, examples of such tools being disclosed in U.S. Pat. No. 3,348,293. The torque applying end may include a handle or, where powerized application is contemplated, a drive shank facilitating coupling to a torque transmitting member, such as the chuck of a drill press or air tool.
The drive end of the mandrel includes a drive tooth which engages against a drive tang formed on the insert, to provide torque transfer to the lead end of the insert.
Typically, the outer surface of the mandrel is threaded to permit the insert to be mounted over the mandrel, the diameter of the threads of the mandrel being substantially less than the inside diameter of the spiral, providing clearance for radial compression of the spiral to fit the smaller diameter of the tapped bore.
In the use of mandrels of the type described, a relatively high incidence of cross threading had been observed, cross threading being the condition in which the spiral does not ride in the tapped grooves but, rather, intersects the same. The tendency toward cross threading is greatest with fine thread inserts, and during the initial and terminal stages of insertion of the spiral into the bore.
During the initial stages, there is a substantial tilting tendency, resulting when the lead end of the insert is attempted to be introduced into the lead portion of the tapped thread. Such tilting is most pronounced where the axis of the mandrel is manually rotated rather than being supported in a mechanical device which assures a precise concentricity between the axes of the mandrel and the workpiece. The tendency toward cross threading is increased by the loose fit of the insert on the driving tool, precluding precise positioning of the insert relative to the tapped aperture, the loose fit being mandated to allow for compression of the coil.
The drive tooth, which must be extremely accurately formed, is designed not only to apply a torsional force to the drive tang but also to prevent pitch variations and cross threading of the lead convolutions of the insert. The latter function is provided by a lip portion which overlaps the lead end of the insert, and constitutes the most fragile portion thereof.
Cross threading at the terminal stages of insertion is sometimes observed, especially when the spiral insert is of a relatively thin gauge wire. During the terminal driving stages, the last convolution of the spiral is unsupported by trailing convolutions and, thus, is susceptible to expansion in an axial direction. Frequently, an insert may be properly inserted and, in the terminal stages, the terminal convolution will become separated or develop a pitch angle different from the remainder of the spiral, leading to a condition in which, due to cross threading, it will be impossible to insert a bolt into the applied insert, necessitating removal and replacement of the insert. Cross threading at the trailing end increases the torque and may result in a jamming or wedging of the components, a condition which may result in fracture of the drive tooth.